Bicycles, tricycles, and similar human-powered vehicles have in the past been provided with propulsion assistors which help the vehicle's operator propel the vehicle with less effort on the operator's part. Examples of such propulsion assistors are found in U.S. Pat. No. 5,755,304 to Trigg; U.S. Pat. No. 5,855,249 to Nishimura; U.S. Pat. No. 6,347,682 to Buchner; U.S. Pat. No. 6,290,014 to MacCready, Jr.; U.S. Pat. No. 6,024,186 to Suga; U.S. Pat. No. 5,865,267 to Mayer et al.; U.S. Pat. No. 5,842,535 to Dennis; U.S. Pat. No. 5,662,187 to McGovern; U.S. Pat. No. 4,906,053 to Kawai; U.S. Pat. No. 4,028,915 to Stahl; U.S. Pat. No. 5,560,442 to Canderle; U.S. Pat. No. 5,341,892 to Hirose et al.; U.S. Pat. No. 5,474,150 to Mabuchi; U.S. Patent Appln. Publication 2002/0147068 to Chikaraishi; German Patent DE4000960 to Stoll; and Brazilian Patent PI 9601936-0 to Tanaka.
A common approach was to provide a roller which frictionally engaged to a vehicle wheel at the wheel's top, with the roller being driven by an electric or internal combustion engine to thereby drive the vehicle wheel. This approach has several disadvantages, e.g., it raises the center of gravity of the vehicle (which can hinder operation), and it is inefficient insofar as propulsion relies on continuously distorting the vehicle's tire.
Another approach has been to add an external cart behind or adjacent the vehicle, with the cart including a motor and serving as the propulsion assistor. This approach also carries disadvantages, e.g., it changes the handling characteristics and effective size of the vehicle.
A more recent approach has been to provide a motor in place of the hub assembly of one of the vehicle wheels. Batteries and controls for this motor are attached to the vehicle's frame. This approach is disadvantageous in that installation and removal of the propulsion assistor is time-consuming: the vehicle is not readily convertible between a solely human-powered vehicle and a propulsion-assisted vehicle. In some cases, it has been proposed to place the battery for the motor in the rotating portion of the wheel. Given the substantial mass of the battery, this increases the rotational inertia of the wheel, degrading vehicle handling and performance.